Yeah, the main downside of dual-classing is the tedium of leveling back up (especially if you're waiting until BG2 and, say, Fighter 13 to dual class). Also, missing out on potential kit benefits in whatever class you're dual-classing to. Kensai/Thief doesn't get poison weapon, or maze traps. Kensage gets fewer spells per day, can't abuse Nahal's, can't be a Sorcerer and skip the memorization rigmarole. Berserker/Cleric...okay, they're fine, but I still like the multiclass better. The benefits of dual-classing may very well outweigh those kit benefits (and depending on your goals, they probably do), but it is a downside.
Level 13 is when Fighters get their other bonus 1/2 an attack, so waiting until then maximizes your APR. This, of course, only applies if you're playing in BG2 thanks to the level cap (even if you removed the level cap and soloed BGEE, I don't think you could hit level 13 Fighter, dual-class, and expect to get your Fighter abilities back without some serious spawn grinding).
It's a power gaming sort of thing. I think it's WAY too long a way to get to the main class. And then you'll have to get to level 14 in the new class before you get your fighter proficiencies back. And you'll still be 1.25 million experience points behind a single class character.
I think third or seventh is a much better place to dual from.
I don't remember for all the classes, but I do know that thieves get a 10th hit dice; after that they get 2 hp per level. Warrior type classes get 3 hp/level after 9, and wizards get 1. Clerics and druids also get 2. But I'd have to get my hands on my 2nd Edition books to be more specific, it's been a while. lol
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to coin a phrase...want to maze an enemy? there's a spell for that
Edit: And at any rate, saying "there's a spell for that" when I'm comparing a single-classed Thief to a Kensai/Thief is a bit of a non sequitur.
still a thief>mage is a character that can enhance his backstabs, much like kensai>thief. but yes point taken. and bounty hunter traps are supreme
I think third or seventh is a much better place to dual from.
1 million xp isn't that big of a deal.
_IF_ I am dualing at 13, I'm power gaming, and if I'm power gaming, I'm soloing. So, maybe that's why it doesn't seem too hard.